Prior owner says that the Closed-Loop fresh water cooling system should be drained and refilled every second year. (In the olden days I did that with all of my cars ... no longer seems necessary.

He gives this description of draining proceedure:

a) With the engine at operating temp, but NOT too hot, slowly remove the cap on the expansion tank.
b) remove the pressure cap.
c)Loosen 3 drain plugs of the (1)engine, (2)air cooler, (3)heat exchanger. Need a small jar and a 1 gallon jug for the waste.
d) use a 50/50 mixture of automobile anti=freeze and water to refill 1/2 gallon each. Quick answer is 1 gallon. The YANMAR manual says that the cooling water capacity is 2.9 L. I personally dont believe that is equal 1 gallon, based on the flush capacity of a modern urinal ie, 1G/3.8L.

Have fun ... Its all part of the adventure